WO 2004/068389 discloses, inter alia, a method of forming a conductive metal region on a substrate that involves depositing on the substrate, e.g. by inkjet printing, an activator such as a catalyst or catalyst precursor (e.g. palladium acetate) that activates reaction of a solution of metal ions and a solution of a reducing agent to form a conductive metal region on the substrate. In a typical embodiment an activator ink comprising palladium acetate dissolved in a mixture of diacetone alcohol and methoxy propanol, and also including polyvinyl butyral and potassium hydroxide, is deposited on a substrate by inkjet printing. After drying of the printed activator ink, one or more solutions including copper sulphate and formaldehyde are inkjet printed onto the activator-containing layer for reaction to form a conductive copper layer.
PCT/GB2004/004589 (WO 2005/045095) discloses methods of forming a solid layer, e.g. of conductive metal, on a substrate, involving use of an activator-containing layer on the substrate. The layer includes a first chemical functionality, e.g. a polymer reaction product of one or more curable monomers and/or oligomers, which is at least partially insoluble in a liquid applied to the layer. The layer preferably also includes a second chemical functionality such as polyvinyl pyrrolidone (PVP), which is at least partially soluble in the applied liquid.
PCT/GB2004/005088 (WO 2005/056875) discloses methods of forming a solid layer, e.g. of conductive metal, on a substrate involving the use of an activator-containing layer on the substrate. The activator is applied in a curable composition, e.g. comprising UV-curable acrylates and/or methacrylates.
The methods of WO 2005/045095 and WO 2005/056875 may be used, inter alia, in the selective deposition in patternwise manner of conductive metals by electroless plating. Applications of selective metal deposition include, but are not limited to, electrical circuitry, antennas, conductors used in displays, heater elements, interconnects and decoration. In the majority of such applications it is of great interest to have a high degree of control on the size and form of features which are patterned and on the physical properties of the metals formed e.g. the reproducibility of the resistance of a particular metal feature.
A common problem in the deposition of solid materials via an intermediate liquid phase is the so-called “coffee stain” effect. This phenomenon occurs when solid layers are produced from solutions by allowing solvent or carrier to be driven off. A combination of surface tension and thermal effects causes the solid component of a solution to be segregated to the edges of a drop of the solution as the solvent evaporates and the drop dries. This gives rise to a solid layer which has a characteristic ring of thicker solid surrounding its perimeter. In situations where the final solid material consists of a blend of several solid materials, this thicker perimeter ring will often have a different composition to the more even centre section of the material. Visually, this may manifest as a difference in colour or tone of the solid layer. If the solid layer is utilised for its other physical properties, such as chemical reactivity, electrical or semiconducting properties, then this effect can lead to a substantial variation in performance across an area of deposited material.
In the case of a solution which contains a catalytic component to promote electroless deposition, this coffee stain effect may give rise to an uneven distribution of catalytic material and hence a variation in the rate of electroless deposition. In most cases the catalytic material will be concentrated in the thicker coffee stain region, resulting in a higher plating rate in this area and hence an enhancement in the thickness variation across a deposited feature. In the majority of cases this is undesirable.